In many types of semiconductor production tools, it is very important to align the semiconductor wafer with respect to internal positioning and/or imaging systems. The alignment usually requires calculation of two main parameters: wafer orientation and wafer center displacement. Wafer orientation is usually measured with the help of a marker on the wafer, such as a flat or a notch, which is oriented to certain crystallographic axes on the silicon substrate.
Reference is now briefly made to FIGS. 1A, 1B and 1C. FIGS. 1A and 1B illustrate wafers with a notch 10 and a flat 12, respectively, and FIG. 1C details the structure of a notch, as defined by the Semiconductor Equipment and Materials International (SEMI) standards for wafer shape. FIG. 1A has a circle 14 indicating the outer periphery on the wafer within which processing or inspection can occur. The small notch 10 is present outside of the circle 14 and is aligned to an orientation or fiducial axis 16 of the wafer. FIG. 1B indicates that the flat 12 is also aligned to the orientation axis 16.
FIG. 1C indicates that the notch 10 has two lines 20 and 22 at 90.degree. to each other and the angle therebetween is bisected by the orientation axis 16. The two lines 20 and 22 do not meet at a point; rather, they are joined by a curve 24.
There are two common approaches for wafer alignment in semiconductor manufacturing.
The first approach recognizes the pattern features, such as scribe lines, on a wafer. Such a method is described in U.S. Pat. No. 5,682,242 and is based on the fact that the patterns laid down on the wafer are accurately aligned to the center and axes of the wafer. This method generally allows successful and accurate alignment except when optical contrast or resolution is not good enough to recognize the pattern features or when a non-patterned wafer, such as a test or a monitor wafer, is to be aligned.
Alignment of non-patterned wafers is often required during production and processing, especially for metrology and inspection equipment, because, in some processes, e.g. chemical-mechanical polishing (CMP), the accurate location of the measurement point is important for recognition of inherent process non-uniformity.
The second and most common approach requires an additional station prior to the measurement or inspection station of the same equipment. This is shown in FIG. 2 to which reference is now briefly made.
Production tools, such as a measurement tool, typically have three stations, a cassette station 30 for receiving the cassettes from another piece of equipment, a pre-aligner 32 which orients the wafer by finding the marker (i.e. notch or flat), and a measurement unit 34 which performs the measurement operation. A robot 36 carries the wafer to be inspected from the cassette station 30 to the pre-aligner 32, where the wafer is oriented, and then to the measurement unit 34.
One prior art notch finder, described in U.S. Pat. No. 5,438,209, is shown in FIG. 3, to which reference is now made. FIG. 3 is a copy of FIG. 2 of U.S. Pat. No. 5,438,209.
The wafer W is placed on a table 1 and is step-wise rotated by a stepper motor 2. A CCD line sensor 3 is placed along the edge of the wafer W and below it for detecting the outline of the wafer W. A light source 4 and an optical system 5 are arranged above the wafer W to illuminate the wafer and the CCD line sensor 3. In addition, the system contains image processing and CPU units which are not presented in FIG. 3.
Stepper motor 2 initially rotates the wafer with a relatively broad pitch and a relatively high speed. Since the image received by the CCD sensor 3 changes when the notch passes the detection means, the notch region is identified. Thereafter, the stepper motor 2 slowly rotates the wafer with a fine pitch so that the outer peripheral positional data in the vicinity of the notch region can be finely sampled. The position of the notch is computed from the outer peripheral positional data and defines, in turn, the wafer's orientation and its center position.
This approach has two main disadvantages. The first is that moving the wafer from the pre-aligner 32 (FIG. 2) to the measurement station 34 causes loss of accuracy due to the mechanical tolerances of any delivery system, such as robot 36; the accuracy of the robot 36 is typically less than that of the pre-aligner 32 which is based on image processing. The second disadvantage is that the additional station requires additional footprint which is not always available, especially when the metrology tool is to be integrated within a processing unit.